Lithium amide is a highly reactive inorganic base widely used in anhydrous organic synthesis. Due to its strong basicity and low nucleophilicity, it is particularly effective for the deprotonation of weak C–H acids and plays a key role in stereoselective transformations, including chiral resolution strategies in the preparation of natural product analogs.
Chemical Properties
Lithium amide (LiNH2, molecular weight 23.00 g/mol) is typically obtained as a white to off-white hygroscopic solid with a cubic crystal structure (a = 5.01 Å). In this structure, Li+ cations are tetrahedrally coordinated by four NH2− anions, with Li–N bond distances of approximately 2.00 Å.
It reacts vigorously with water according to the following reaction:
LiNH2 + H2O → LiOH + NH3
This reaction leads to the rapid formation of lithium hydroxide and the release of ammonia gas. Under thermal conditions, lithium amide remains stable up to approximately 380°C, above which it converts to lithium imide (Li2NH).
Biochemical Applications
In synthetic carbohydrate chemistry, lithium amide is commonly used in stoichiometric amounts (1–2 equivalents in tetrahydrofuran, THF) to achieve regioselective deprotonation of equatorial C–H bonds in glucal derivatives. This enables efficient C-glycosylation reactions without the need for directing groups.
In nucleotide chemistry, lithium amide facilitates the generation of 5′-O-anions in protected nucleosides. Its strong basicity (pKa ≈ 38) allows access to N–H and C–H acidic sites that are not readily deprotonated by weaker bases such as sodium amide (NaNH2).
Furthermore, chiral lithium amide derivatives, particularly those prepared from (R)- or (S)-BINOL ligands, are widely employed in asymmetric synthesis. They enable the resolution of racemic amino alcohols through diastereoselective kinetic protonation, contributing to the synthesis of enantiomerically enriched compounds.